The present invention generally relates to image transfer in digital imaging systems. In particular, the present invention relates to an efficient approach for reducing image transfer time in wireless portable x-ray detectors.
Digital imaging systems may be used to capture images to assist a physician in making an accurate diagnosis. Digital imaging systems typically include a source and a detector. Energy, such as x-rays, produced by the source travel through an object to be imaged and are detected by the detector. An associated control or image processing system obtains image data from the detector and prepares a corresponding diagnostic image on a display.
The detector may be an amorphous silicon flat panel detector, for example. Amorphous silicon is a type of silicon that is not crystalline in structure. Image pixels are formed from amorphous silicon photodiodes connected to switches on the flat panel. A scintillator is placed in front of the flat panel detector. For example, the scintillator receives x-rays from an x-ray source and emits light of an intensity related to the amount of x-rays absorbed. The light activates the photodiodes in the amorphous silicon flat panel detector. Readout electronics provide pixel data or values from signals from the photodiodes through data lines (columns) and scan lines (rows), for example. Images may be formed from the pixel data. Images may be displayed in real time. Flat panel detectors may offer more detailed images than image intensifiers. Flat panel detectors may allow faster image acquisition than image intensifiers depending upon image resolution.
A solid state flat panel detector typically includes an array of picture elements (pixels) composed of Field Effect Transistors (FETs) and photodiodes. The FETs serve as switches, and the photodiodes are light detectors and image storage elements. The array of FETs and photodiodes may be composed of amorphous silicon. A compound such as Cesium Iodide (CsI) is deposited over the amorphous silicon. CsI absorbs x-rays and converts the x-rays to light. The light is then detected and stored by the photodiodes. The photodiode acts as a capacitor and stores the charge.
Initialization of the detector occurs prior to an exposure. During an initialization of the detector, the detector is “scrubbed” prior to an exposure. During scrubbing, each photodiode is charged to a known bias voltage that represents “black”, or no light output. The detector is then exposed to x-rays which are absorbed by the CsI deposited on the detector. Light that is emitted by the CsI in proportion to x-ray flux causes the affected photodiodes to conduct, partially discharging the photodiode.
After the conclusion of the x-ray exposure, the voltage on each photodiode may be gated through a FET switch to readout electronics that may include, for example, an analog voltage comparator, which compares the photodiode's stored voltage with a voltage generated from a digital to analog (D/A) converter. The digital input to the D/A converter may begin at ‘0’, for example, and is incremented through a programmable ramp to a maximum value. As the analog ramp increases on the output of the D/A converter, the output eventually equals or exceeds the voltage coming from the photodiode, at which time the analog voltage comparator latches the current value of the D/A converter, which may represent, for example, the digital pixel value for that photodiode. As another example, the comparator may output the value of input to the D/A converter. This input may be, for example, an index value for the programmable ramp. That is, the programmable ramp may be generated based at least in part on an index value provided as an input to the D/A converter.
The readout electronics may then use a lookup table (or other data structure) to translate the output of the comparator (e.g., the index value) to a pixel value. Because it is desirable for the conversion of the signal from the detector element to a digital pixel value to occur as quickly as possible, the total number of index values input to the D/A converter may be much less than the range of the ramp values. As an example, the index value may range from, for example, 1-1800, while the corresponding ramp values may range from 1-16,000. To convert the index value output by the comparator to a pixel value, which may be, for example, the ramp value, the lookup table may contain entries for each index value (e.g., 1-1800) mapping each to a corresponding ramp value (e.g., 1-16,000). Although the ramp value latched may not exactly equal (i.e., as mentioned, it may exceed) the detector element's signal, this method allows for a much faster conversion from the analog detector signal to the digital value. Thus, the energy detected by the detector element is converted from an analog voltage to a digital pixel value.
In any imaging system, x-ray or otherwise, image quality is important. In this regard, x-ray imaging systems that use digital or solid state image detectors experience certain electrical phenomena that may cause imaging difficulties. Difficulties in a digital x-ray image may include image artifacts, “ghost images,” or distortions in the digital x-ray image. Imaging difficulties may be caused by effects such as electronic current leakage from imaging system circuitry, x-ray detector, and the like. During x-ray system calibration, a “dark” image may be acquired to adjust the image intensity offset. A “dark” image is a reading taken of the image intensifier, CCD, flat panel detector, and the like, without x-ray exposure. For example, a “dark” image may be acquired from a flat panel detector when no x-rays are being emitted from the source. By way of example, one electrical phenomena is that, over time, electronic circuits experience drift in their baseline response and changes in their gain response. Changes in baseline response and gain cause an “offset” or change in the electrical response of the detector for the signal produced based on a given x-ray count. For example, a new detector may produce a 5 volt signal when an x-ray count of 5000 RADs is detected. However, as time passes, the baseline response may increase 5 volts and thus the detector may produce a 10 volt signal when the same 5000 RAD count is detected. A “dark” image may determine the offset produced by the detector and x-ray system since it will capture the baseline shift. By subtracting the “dark” image pixel values from the actual “exposed” x-ray image pixel values of a desired object (i.e., the “light” image), the offset effects may theoretically be eliminated. Conventional systems may acquire offset readings in between x-ray imaging exposures. That is, a “dark” image and a “light” image may be acquired as pairs. During, for example, system calibration, sequences of “dark” images may be acquired without intervening “light” images.
Additionally, many imaging products are mobile, which offers hospitals, clinics, and physicians the ability to move these systems from room-to-room or to bring x-ray capability to a patient that cannot be moved. However, a portable imaging system may require one or more wires or cables from, for example, the detector to a control or image processing component of the system. These wires may encumber placement or movement of the system. As another example, the length of the wire may limit where the portable system may be used. As another example, the wires may become tangled up with the portable components and/or their operators.
These problems with cables may be resolved, in part, by using wireless communication in the imaging system. For example, wireless communication may be used between the detector and the host processing component. However, a major challenge with a wireless detector is image transfer speed. Because wireless communications generally have significantly slower transfer speeds than wired communications, transferring an image may require an unacceptable amount of time. For example, for a “light” image that is 2048 pixels by 2048 pixels, with each pixel being represented by a 16 bit value, 64 Megabits (Mb) must be transferred for the image alone, not including any communications overhead. A wired system may have a bandwidth of 100 Mb/s, for example. Therefore, roughly 0.64 seconds would be required to transfer the image (again, not taking into account communication overhead). For a wireless system with a bandwidth of, for example, 11 Mb/s, almost 6 seconds would be required for the transfer. This much greater transfer time may be unacceptable to users of the system. Further, as mentioned, this transfer time does not account for communications overhead which may be, for example, 30%, increasing the transfer time even more. Thus, it is highly desirable to reduce image transfer time in wireless portable detectors.
Therefore, there is a need for an efficient approach for reducing image transfer time in wireless portable x-ray detectors.